Lifting magnets have been used for decades for lifting magnetically permeable material without the need for physical attachment to the object, for example, by cables or chains. Steel slabs are moved in this manner, and also scrap iron is moved, for example, from a railroad car to a pile of scrap to be used in a melting furnace in a steel mill. Such lifting magnets typically have had a central pole piece surrounded by a coil and an outer peripheral pole piece within a magnet assembly, generally of a flat disc shape, supported by chains on a lifting crane. Such lifting magnets had a large annular air gap between the central pole piece and the peripheral pole piece, and this air gap is designed to be bridged by the object or objects to be lifted. Where such object is a flat object, such as a steel slab, a good surface-to-surface contact may be made between the lifting magnet pole pieces and the steel slab object, and therefore the magnet assembly may lift considerably more weight than if the object is scrap iron. Scrap iron, made of pipe or other objects of irregular shape, will not have a good area of contact at the pole pieces, but generally only a line contact. This prevents such lifting magnet from lifting as much weight of scrap iron as that of steel slabs.
The magnet case for lifting magnets has traditionally been made from a cast material, such as cast steel, as shown in U.S. Pat. Nos. 928,510 and 1,015,728. This cast steel upper portion of the magnet case provides protection to the winding, provides a tapering flux path tapering to a thinner cross section at the radially outer edges, and provides a strong structure which will withstand battering of the magnet by the lifting magnet crane operator during use.
It was long ago realized that it was wasteful of cast steel material and crane lifting energy to have a constant thickness top plate on the magnet case because the greatest flux density in such top plate was adjacent the central core. It was realized one could keep generally uniform flux density throughout the radial dimension of the magnet case top plate by making this top casting gradually tapering to a smaller thickness at the outer periphery. This tapering shape was generally shown in U.S. Pat. Nos. 1,334,504, 1,459,830 and 4,112,248.
Lifting magnets fabricated from steel plate have also been suggested, for example, as in U.S. Pat. No. 3,984,796. Such lifting magnet assembly had a steel plate, for example, of cold-rolled steel, which had uniform thickness and was utilized for the top plate of the magnet case. This was inefficient in the weight versus lifting capacity because the uniform thickness top plate would have a high flux density near the central core and a low flux density at the outer periphery. It, therefore, had been previously suggested to utilize a stack of annular, uniform-thickness plates welded together, successive plates becoming smaller in outer diameter to thus achieve a fabricated top plate which had greater thickness near the central core and lesser thickness at the outer periphery.
The fabricated magnets of a plurality of steel plates were one solution to the problem of obtaining good castings of steel to be used for the magnet case on the lifting magnets. Typically, such steel castings are relatively complex and, due to their large size, a part of the sand mold or core might break loose during pouring of the molten steel, thus making a defective steel casting. Also, lifting magnets were desired in a wide range of lifting capacities, e.g., from 300 pounds to 17,000 pounds, which took a very large number of diameters of lifting magnets to achieve this lifting capacity range. Each one took a different size casting, which increased the manufacturing cost.